Sorry - mistakenly thought you'd seen it in Edinburgh for some reason. As other posters have said, if it was a CS liveried 92 in Euston that morning I think it could only be 92014 as 010 and 038 were in Scotland at the time and the other 3 are in Brush Loughborough.
From what I can tell, 92014 brought the Lowlander down from Glasgow Tue night, arriving Euston Platform 1 at 06:28 Wed morning:
http://www.realtimetrains.co.uk/train/C74099/2017/01/03/advanced
They'd have then detached 92014 and a light loco (e.g. 87002) brought in and hooked on to the North end to take the ECS back to Wembley (d. 08:26). 92014 is left behind and parks up on the blocks at Euston.
The Highlander with you on board then comes into Platforn 15 at 08:06 hauled by 90048:
http://www.realtimetrains.co.uk/train/C74102/2017/01/03/advanced
92014 then leaves its blocks, heads out of the station and comes back in on P15 to hook up to the North end of the Highlander and then takes the ECS (with 90048 still on the back) back to Wembley departing 09:26...
http://www.realtimetrains.co.uk/train/C74127/2017/01/04/advanced
I'm filling in a few gaps here, so may not be 100% accurate, but this is a typical movement (as saves bringing in an extra loco) and would explain how you saw 92014 on the blocks when you arrived, but then also saw it coming in light loco when you left on the Overground - as it would have left the station where it'd been parked up, gone "round the corner" just out of sight, then come back in to get the Inverness ECS.